Acid value, hydroxyl value, and isocyanates in raw materials for the fabrication of plastics

Importance of the topic

Polyurethanes and related alkyl or polyester acrylate resins form the backbone of many industrial plastics. Accurate determination of their acid value, hydroxyl value, and isocyanate content is critical for ensuring consistent product performance, optimizing stoichiometry in polymer synthesis, and maintaining batch-to-batch quality in manufacturing.

Objectives and study overview

This bulletin presents automated potentiometric titration protocols for:

• Acid value measurement according to ASTM D4662 and ASTM D7253
• Hydroxyl value determination via ASTM E1899 and DIN 53240-3 methods
• Isocyanate (NCO) content assessment following EN ISO 14896

The goal is to demonstrate fast, reproducible assays with minimal sample preparation for raw materials used in polyurethane production.

Used instrumentation

Key components include:

• Automated sample changer and magnetic stirrer
• Potentiometric titrator operating in differential electrode test (DET) mode
• Various metering burets (2, 20, 50 mL) for titrant, solvent, and reaction solution delivery
• Solvotrode easyClean pH electrode
• DIS-Cover titration vessel cover to prevent evaporation and contamination

Methods and instrumentation

Each assay follows a standardized workflow: sample dissolution in appropriate organic solvent or solvent mixture, addition of indicator or reaction reagent (e.g., TSI for hydroxyl groups), controlled reaction time, and potentiometric titration to the first or second equivalence point. Parameter settings (signal drift, waiting time, endpoint recognition) are optimized for rapid detection of inflection points.

Main results and discussion

Typical titration curves exhibit well-defined endpoints with minimal drift. Calibration with standards such as benzoic acid (acid value), potassium hydrogen phthalate (hydroxyl value), and TRIS (isocyanate assay) yields titers with <1 % deviation. Automation reduces operator error and allows throughput of up to dozens of samples per day with full data logging.

Benefits and practical applications

Automated potentiometric titration delivers:

• High accuracy and precision across multiple standards
• Short analysis times (12 min for hydroxyl vs. 40 min in manual DIN method)
• Reduced solvent and reagent consumption
• Scalable QC for routine production and R&D labs

Future trends and potential applications

Advances may include inline process monitoring using flow-through electrodes, greener solvent systems to minimize environmental impact, miniaturized titration cells for field analysis, and integration with LIMS for fully automated polymer quality control.

Conclusion

The described automatic potentiometric titrations provide robust, efficient, and reproducible methods for assessing critical functional group concentrations in polyurethane raw materials, supporting product consistency and process optimization.

References

• ASTM D4662: Acid and Alkalinity Numbers of Polyols
• ASTM D7253: Acid Number for Polyether Polyols
• ASTM E1899: Hydroxyl Groups via TSI Reaction
• DIN 53240-3: Rapid Test for Hydroxyl Value
• EN ISO 14896: Isocyanate Content in Polyurethane Raw Materials
• ASTM D5155 and ASTM D2572: Alternative Isocyanate Determinations